Along with a sudden increase in the amount of data to be dealt with in the field of magnetic disc, the increase of the capacity of floppy discs has been demanded.
Further, in the field of magnetic tapes also, with the spread of office computers, such as minicomputers, personal computers and work stations, magnetic tapes for recording computer data as external storage media (so-called backup tapes) have been eagerly studied in recent years. For putting magnetic tapes for such usage to practical use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing ability (e.g., information throughput).
Magnetic heads working with electromagnetic induction as the principle of operation (an induction type magnetic head) are conventionally used and spread. However, magnetic heads of this type are approaching their limit for use in the field of higher density recording and reproduction. That is, it is necessary to increase the number of winding of the coil of a reproduction head to obtain larger reproduction output, but when the winding number is increased, the inductance increases and the resistance at high frequency heightens, as a result, the reproduction output lowers. In recent years, reproduction heads which work with MR (magneto-resistance) as the principle of operation are proposed and come to be used in hard discs. As compared with the induction type magnetic disc, several times of reproduction output can be obtained by the MR head. Further, since an induction coil is not used in the MR head, noises generated from instruments, e.g., impedance noises, are largely reduced, therefore, it becomes possible to obtain a great S/N ratio by lowering the noise coming from magnetic recording media. In other words, good recording and reproduction can be done and high density recording characteristics can be drastically improved by lessening the noise of magnetic recording media hiding behind the instruments.
There is disclosed in JP-A-10-340445 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) a magnetic recording medium having an upper magnetic layer which shows excellent durability, electromagnetic characteristics, in particular, excellent output and overwriting characteristics by setting the thickness of an upper magnetic layer at 0.5 μm or less to suppress the fluctuation at interface between a magnetic layer and a nonmagnetic layer. For suppressing interfacial fluctuation, the same patent adopts wet-on-dry coating of coating a nonmagnetic lower layer and then coating an upper magnetic layer after the nonmagnetic lower layer has been dried. However, the thickness of 0.3 μm of a magnetic layer prescribed in the patent is too thick and is not suitable for higher density recording. Further, in general, the durability of a magnetic recording medium produced by wet-on-dry coating is liable to be deteriorated, and this tendency is more conspicuous when the thickness of a magnetic layer is thin.
JP-A-5-298653 ensures high electromagnetic characteristics and high durability by setting the average thickness of an upper magnetic layer at 0.01 to 0.3 μm to control the fluctuation at interface between a magnetic layer and a nonmagnetic layer so that (standard deviation of a magnetic layer thickness)/(average magnetic layer thickness) becomes within the range of 0.05 to 0.5. However, there is a limit in wet-on-wet coating adopted by the same patent to obtain the magnetic recording medium to suppress the interfacial fluctuation between a magnetic layer and a nonmagnetic layer, and when a magnetic layer is further thinned, the fluctuation at interface unfavorably causes noises. In addition, the laminated Sendust head as used in the patent is an inductive head, but an MR head for use in higher density recording in the future is more susceptible to the influence of a medium noise, hence the interfacial fluctuation will be actualized.
JP-A-2000-173038 ensures high electromagnetic characteristics and high durability by setting the thickness of a magnetic layer at 0.05 to 0.5 μm and adding diamond particles having an average particle size of from 0.05 to 1.0 μm to the magnetic layer in proportion of from 0.1 to 5 mass % (i.e., weight %) based on the amount of the ferromagnetic metal powder. However, the layer thickness in the Examples of the above patent is mainly restricted to 0.3 μm, and it is necessary to make magnetic layer thickness thinner for achieving higher density. Further, the layer thickness of about 0.3 μm is not liable to be influenced by the interface between a magnetic layer and a nonmagnetic layer, and S/N ratio does not deteriorate even by wet-on-wet coating.
The evaluation of the magnetic recording media disclosed in these patents was performed by recording and reproducing using an MIG (metal-in-gap) head which is an inductive type magnetic head and track density is relatively low, and when higher density recording is done by further lessening a track width or thinning the magnetic layer thickness, a sufficient S/N ratio cannot be obtained at reproduction. In particular, the influence of interfacial fluctuation becomes large when an MR head is used, which causes the degradation of S/N ratio.
A low noise and high density magnetic recording medium and also excellent in running durability is desired even when a conventionally used coating-type magnetic recording medium which is excellent in productivity and inexpensive is combined with an MR head.